1. Field of the Invention
The present invention relates to a system for measuring the solubility of solid compounds in supercritical fluids such as methane or carbon dioxide.
The invention relates more particularly to the measurement of the solubility of various solid organic compounds in a supercritical gas (CH.sub.4, CO.sub.2, etc.) in a high pressure range ranging for example between 7 and 150 MPa, and at temperatures that may range between 20.degree. and 200.degree. C.
The system according to the invention allows dynamic solubilization and extraction with on-line or off-line analysis of the sample.
The system according to the invention has applications in many fields where constituents of mixtures are to be separated and more particularly when some of the constituents to be isolated occur in very small quantities, for example in the form of traces. The agrobusiness can for example be cited, where hop flavours used to produce beer are to be isolated, or caffein has to be separated to produce decaffeinated coffee. The invention also has applications in the aeronautical and space industry where very pure components isolated from mixtures or alloys are to be manufactured. Other applications exist in the petroleum, gas, chemical industries where solid heavy hydrocarbons are to be extracted from mixtures for example.
2. Description of the Prior Art
It should be noted that a gas is in the supercritical state when it is at a temperature and at a pressure that are higher than that of the critical point thereof, a fluid state that is neither liquid nor gaseous. The solvent power thereof in this state greatly depends on the temperature and the pressure.
The solubility of solids and of liquids in supercritical fluids can be determined by means of two different experimental techniques: the synthetic method, by means of phase equilibria, and the analytical methods requiring sampling and analysis.
1) The synthetic method is based on the visualization of the phase changes of solid-fluid or liquid-fluid binary mixtures in a thermostat-controlled high-pressure cell provided with a window (generally made of sapphire or quartz) whose volume can be varied through the displacement of a piston moving in a chamber. The cell is charged with a known quantity of solid or liquid compound. After several purges with gas (CO.sub.2, methane) intended to draw the entrapped air away, a known quantity of gas is thereafter introduced into the cell. The phase changes of the mixture can then be observed by means of a camera placed against the window.
At a given temperature, the supercritical solid-fluid mixture contained in the cell is slowly compressed until the solid is solubilized by the fluid, a single phase being then present in the cell. The pressure is then slowly decreased until two phases appear. The mixture is alternately compressed and decompressed several times so that the pressure range for which a phase change is observed is as small as possible. Solubility in this pressure range is readily determined because the quantities of solid and of gas introduced in the cell are known with precision.
The synthetic method both presents advantages and drawbacks insofar as phase transitions are detected visually, the solubilities of the liquids and of the solids are obtained without sampling, the quantities of compound and of supercritical fluid used are small, and the devices used to implement the method are sophisticated and therefore expensive. An example of application of the synthetic method is described for example in:
McHugh M. et al: "Solid Solubilities of Naphtalene and Biphenyl in Supercritical Carbon Dioxide", J. Chem. Eng. Data, 1980, 25, 326-329.
2) Analytical methods are more commonly used to study solubility in supercritical fluids. The implementation devices can be used either to extract one or more solid compounds from a matrix, or to study the solubility of a solid compound in the supercritical fluid. In this case, the cell contains only the compound whose solubility is to be measured.
Implementation devices generally comprise in combination a solvent tank, a pump, an extraction cell, a recovery system and an analysis system. Their design, selection of the recovery system and of the analysis method differ according to the aim and to the solutes studied.
The main three analytical methods known for solubilization by supercritical fluids are the static method, the dynamic method and the semi-dynamic method halfway between the previous two methods.
2.1) The static method consists in bringing the compound into contact with a given volume of fluid. The supercritical fluid is introduced, then maintained in the extraction cell and mixed by stirring with the compound to be solubilized. The temperature is kept constant by a thermostat-controlled bath or a furnace. Prolonged contact between the fluid and the compound guarantees good equilibrium and therefore solubilization conditions. The quantity of supercritical fluid required is limited but the solubilization time required is longer and generally ranges from one to three hours. An example of use of the static method is described for example in:
Hollar W. P. et al: "Solubility of Naphtalene in Mixtures of Carbon Dioxide and Ethane", J. Chem. Eng. Data, 1990, 35, 271-275.
2.2) The dynamic method is most often used to measure the solubility in a supercritical fluid that is for example brought continuously in contact with the compound to be solubilized. The implementation device can be an open-circuit device, the pure supercritical fluid circulating through the compound to be solubilized. The flow rate of the supercritical fluid imposes the velocity of flow of the fluid through the compound, and therefore the contact time. The geometry of the solubilization cell and the characteristics of the solid compound (grain size) influence the search for equilibrium conditions. An example of implementation of an open-circuit device is described for example in:
Van Leer et al: "Solubilities of Phenol and Chlorinated Phenols in Supercritical Carbon Dioxide", J. Chem. Eng. Data, 1980, 25, 257.
In the dynamic mode, the solubilization time is shorter than in the static mode but the quantity of fluid used is greater.
2.3) In the semi-dynamic method, the previous two operating methods can be combined by prolonging, according to a dynamic mode, a short static period performed at the solubilization start when equalizing the pressures and the flow rate. Combination of these two modes allows improvement of the supercritical solid-fluid equilibrium and therefore to obtain quite readily solubility equilibrium, and it requires shorter solubilization times than the static mode. Such a semi-dynamic method is for example described in:
Yau J. S. et al: "Solubilities of Heavy n-Paraffins in Subcritical and Supercritical Carbon Dioxide", J. Chem. Eng. Data, 1993, 38, 171-174.
Analytical methods afford certain advantages in relation to synthetic methods. The cost of the equipment used is moderate. The experimental device can also be used for extraction. Sampling in the supercritical state prevents analysis errors due to precipitation in the relief valve.
3) Extract Recovery
After the solubilization process, a sampling has to be achieved in order to determine the solute concentration and to measure the volume of solvent gas. Sampling can be performed in the supercritical state or by expansion. According to the configuration, the sampling and analysis stages can be coupled, analysis being then performed on-line or off-line. The volumes sampled are very different according to the coupling type and to the analysis method.
In the supercritical state sampling mode, the supercritical fluids being gaseous at atmospheric pressure, the compound can be precipitated by simple expansion. The flow rate of the supercritical fluid and the expansion method influence the recovery percentage. Too high a flow rate can lead to losses upon expansion, especially for volatile solutes. The great majority of the devices are equipped with a HPLC type sampling valve for example which imposes the flow rate of the fluid, and therefore the pressure in the cell. This valve is heated in order to prevent precipitation of the solutes that might modify the flow rate upon expansion. Several collection modes can be envisaged. The compound can be recovered by cryogenic trapping in a cooled empty tube in order to increase the trapping efficiency and to prevent carry-over of the volatile compounds. The higher the flow of gas, the lower the required temperature. The recovery ratio is improved by adding for example an adsorbent in the tube or by using a cooled chromatographic column. The compound can also be collected by bubble-type recovery within the scope of an off-line analysis, or on an adsorbent after expansion. The extracts precipitate and are trapped on a stationary phase. This type of recovery has a better reproducibility than bubbling; the flow rates used can be higher, hence a greater analysis speed.
4) Analysis
a) Analysis can be performed off-line for solubility studies since the sample is to be weighed, and also on-line with quantitative transfer of all of the compound sampled, which prevents pollution and guarantees high sensitivity, hence high accuracy.
Supercritical sampling can be directly coupled with various gas, liquid or supercritical chromatographic techniques and common detection methods can be used. The most delicate stage during coupling is the recovery of the solute prior to analysis and therefore the separation of the solute and of the solvent. Various techniques of coupling with gas, liquid or supercritical phase chromatography are described for example in:
Hawthorne et al: Directly Coupled Supercritical Fluid Extraction-gas Chromatographic Analysis of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls from Environmental Solids--Journal of Chromatography, 1987, 403, 63-76;
Unger K. K. et al "On-Line High-Pressure Extraction-High-Performance Liquid Chromatography--Journal of Chromatography, 1983, 282, 519-526, or
Jackson W. P. et al: Supercritical Fluid Injection of High-Molecular-weight Polycyclic Aromatic Compounds in Capillary Supercritical Fluid Chromatography, in J. HRC & CC, 1986, 9, 213-217.